If you throw 2 dice, how large is the possibility that they show the same number?

The singular is die, the plural is dice. A die might have various numbers of faces, so it depends on what kind of dice you are talking about.

Assuming you mean the familiar six sided version, there are six numbers and 36 combinations of two dice, so the chance of same numbers are 1 in six and the chance of different numbers are 30 in 36 or 5 in 6.

'Chance' and 'odds' are not the same thing. Odds refers to what a bookie will offer to pay a winning bet, which may be somewhat different from the chance you will win.